It is necessary to drill many oil and gas wells utilizing turbines and mud motors. Both turbines and mud motors are devices utilizing drilling fluid, normally known as drilling mud, as the flowing medium which rotates the drive components of the motor. Drilling fluid is normally formed of materials which, in water or oil base include solids that form an abrasive fluid. The abrasive fluid will destroy bearing assemblies. This is especially true of precision thrust or roller bearing assemblies. When they are enclosed in a sealed container, improved life can be obtained. By contrast, the present disclosure is directed to an exposed bearing assembly which does not wear rapidly in the presence of abrasive drilling fluid. The present disclosure sets forth a bearing assembling which is intentionally exposed to the abrasive drilling fluid in the immediate vicinity. More specifically, the bearing assembly is able to operate in the intended fashion notwithstanding the flow of abrasive drilling fluid in the immediate vicinity. One aspect of the flow of the drilling mud in the immediate vicinity of the bearing assembly is the risk of erosion. The fluid flow, especially with abrasive particles carried in it, tends to wear or grind away the exposed bearing assembly. The present disclosure sets forth a method and mechanism for protection of the bearing assembly.
It is not uncommon to form such a bearing assembly with an exposed circular disc of hardened materials which provides the surface for the rotating components. This exposed surface is made of natural or synthetic diamonds bonded to supporting carbide discs. There are several types of diamond wear pads known to the drilling industry. In one type, the diamonds have a large size and are positioned on the surface of a ring in a predetermined pattern. This can use a disc formed of a polycrystalline diamond supported on a sintered carbide support member.
In the following disclosed apparatus, it should be understood that the term polycrystalline diamond, PCD, or sintered diamond, as the material is often identified in the literature, can also be any of the superhard abrasive materials, including, but not limited to synthetic or natural diamond, cubic boron nitride, and wurtzite boron nitride as well as combinations thereof. In like manner, cemented metal carbide refers to a carbide of one of the group IVB, VB, or VIB metals which is pressed and sintered in the presence of a binder of cobalt, nickel, or iron and the alloys thereof.
Diamonds are an allotropic form of carbon, which is crystallized isometrically. It consists of carbon atoms covalently bound by single bonds only in a predominantly octahedral structure. This accounts for its extreme hardness (Mohs 10) and great stability. It has a specific gravity of 1.5 and a coefficient of friction of 0.05. Diamonds melt at 3700.degree. C. They can also be made synthetically by heating carbon and a metal catalyst in an electric furnace at about 3000.degree. F. under pressure of about 1.3 million psi.
Carbide for the ring is a binary solid compound of carbon and another element. The most familiar carbides are those of tantalum, titanium, tungsten, silicon, boron, and iron (cementite). Two factors have an impact on the properties of carbides and they are: (1) the difference in electronegativity between carbon and the second element, and (2) whether or not the second element is a transition metal. A "cemented carbide" is formed from a powdered form of refractory carbide which is united by compression with a bonding material (usually iron, nickel, or cobalt) followed by sintering. For example, tungsten carbide is bonded with 3 to 25 percent cobalt at 1400.degree. C. Cemented carbide is used chiefly in metal cutting tools which are hard enough to permit cutting speeds in rock or metal up to 100 times that obtained with alloy steel tools.
Boron nitride (BN) occurs as a white powder, with a particle size of about 1 micron, having a graphite-like hexagonal plate structure which melts at 3000.degree. C. When compressed at about one million psi, it becomes about one half as hard as diamond. The resulting material has excellent heat and shock resistance.
This bearing assembly is related to composite or adherent multimaterial bodies of diamond, cubic boron nitride (CBN) or wurtzite boron nitride (WBN) or mixtures thereof for use as an abrasion resistant bearing pad particularly used in a mud motor.
Composite polycrystalline diamond compacts, PCD, have been used for industrial applications including rock drilling and metal machining for many years. One of the factors limiting the success of PCD is the strength of the bond between the polycrystalline diamond layer and the sintered metal carbide substrate. This, however, results in a cutting tool with a relatively low impact resistance. Due to the differences in the thermal expansion of diamond in the PCD layer and the binder metal used to cement the metal carbide substrate, there can be a shear stress in excess of 200,000 psi at the interface between these two layers. The structured bond opposed to this stress must be located in an extremely thin layer of cobalt which is the common or preferred binding medium that holds the PCD layer attached to the metal carbide substrate. Because of the very high stress between the two layers which normally have a flat and relatively narrow transition zone, it is relatively easy for the bearing to delaminate in this area upon impact. Additionally, it has been known that delamination can also occur on heating or other disturbances in addition to impact. In fact, parts have delaminated without any known provocation, most probably as a result of a defect within the interface or body of the PCD which initiates a crack and later results in catastrophic failure.
The PCD is normally positioned on the beating ring or assembly in the form of a steel disc. It is constructed and arranged so that it supports substantial weight. For cooling, the mud (typically water and abrasive constituents) flows over this area with such volumetric flow that erosion normally occurs in the area of the support ring between the circular disc of PCD. This erosive impact from flow destroys the bearing assembly prematurely. It cuts through the bearing assembly, removing steel which is required for the body of the bearing assembly and thereby cutting the support for the PCD disc. One aspect of this invention is the use of a tungsten carbide ring to prevent erosion.
Another aspect of the present invention is the incorporation of a curve shoulder or protruding lip adjacent to the PCD disc. This is incorporated for the express purpose of alternating the flow pattern of the drilling fluid. Because the drilling fluid does not directly impinge on the face of the bearing ring, erosion is reduced, perhaps even avoided, and the surrounding shoulder helps shelter the PCD disc which prevents rapid wear of the entire bearing assembly. An important aspect of the present disclosure is therefore incorporation of a surrounding peripheral ring construction with a protruding lip or shoulder which thereby deflects the flow of drilling fluid away from the PCD disc and which reduces erosion. This lengthens the life of the equipment substantially.
One aspect of the present disclosure is the construction of a ring shaped bearing assembly incorporating a surrounding shoulder mounted on the exposed face of the ring assembly wherein the shoulder deflects fluid flow to thereby preserve the bearing assembly during use. The erosive impact of fluid flow is markedly changed. The erosive effect of the drilling mud is substantially reduced to thereby avoid weakening of the ring shaped bearing assembly between adjacent PCD discs The structure of this disclosure thus incorporates a round bearing assembly formed of a hardened carbide noted above, the preferred being a matrix supporting tungsten carbide ring, and an attached or integral adjacent ring on the exposed face. The ring defines a shoulder which extends sufficiently high above the face of the ring so that the PCD discs mounted on the ring are protected from erosion.
For purposes of use in a turbine or mud motor, the product of the present disclosure is a ring shaped bearing assembly wherein the ring has a cement carbide body, rectangular in cross-section, having a brazed set of PCD discs thereon. In addition to the brazed discs, an adjacent ring defining an upstanding shoulder may be incorporated to protect against the erosive impact of use. Moreover the attached ring defining the upstanding shoulder is integral or brazed on the base ring. This fabrication step can be carried out in conjunction with the necessary fabrication steps required to attach the several discs to the ring.